1. Field of the Invention
The present invention relates to an on/off-track detecting apparatus that determines whether its information pickup point is on a recording track (pit surface), i.e., in an on-track state, or between recording tracks (mirror surface), i.e., in an off-track state. More particularly, the present invention relates to an on/off-track detecting apparatus for use with a compatible disk player capable of playing different types of information-recorded disks.
2. Description of the Related Art
A currently marketed disk player capable of playing an information recording disk such as a digital audio disk called compact disk (CD), a video disk (VD), etc. is generally equipped with what is known as a track jump function. This function that allows the information pickup point on the disk to skip a plurality of tracks at a time before reaching a desired location. With track jumps involving tens or hundreds of tracks to be skipped, settlement of the tracking actuator takes a relatively long time for because the tracking error signal is liable to exceed a linear operating range of the tracking servo system. To suppress an instable state after each jump, the tracking servo loop includes a braking circuit. This braking circuit applies brakes on the tracking actuator by turning on and off a loop switch for the tracking servo loop at appropriate timings. To determine when to turn on and off the loop switch, the braking circuit requires an on/off-track signal. The signal indicates whether the information pickup point is on a track (on-track state) or off a track (off-track state).
FIG. 1 is a block diagram of an on/off-track detecting circuit. With CD players, as illustrated, an EFM (eight-to-fourteen modulation) signal is read from the disk, forwarded through a DC cut-off capacitor C, and amplified by an amplifier 31. The signal then undergoes a peak and bottom hold operation by a peak and the bottom circuit 32. A difference between the peak and the bottom hold signal is produced by a differential amplifier 33, which provides an envelope signal of the EFM signal. Meanwhile, a hold signal is obtained by a hold amplifier 34 which equals two-thirds of the peak value of the envelope signal. This hold signal is compared in level with the envelope signal by a comparator 35. The result provides an on/off-track signal that goes Low in an on track state and High in an off-track state.
With VD players, the output signal of the pickup varies between on-track and off-track state. For this reason, moving the information pickup point across tracks during a track jump causes the output signal of the pickup to form a sine waveform. FIG. 2 shows another on/off-track detection circuit. As illustrated, the output signal of the pickup is fed to a low-pass filter 41 for removal of the RF component. The output from the filter is fed to a comparator 42. There, the output is compared with a predetermined reference voltage Vref for waveform shaping so as to produce an on/off-track signal.
Currently marketed compatible disk players are capable of playing different kinds of disks such as compact disks and video disks. In these players, if two on/off-track detection circuits are provided for CD and VD use, the circuit configuration becomes complex and the costs thereof are correspondingly high. However, it is impossible to use the VD-use on/off-track detection circuit to detect on-and off-track states through the EFM signal. Similarly conventional CD-use on/off-track detection circuits could not be applied to the RF signal (read from video disk), since the RF frequency is too high for the peak and bottom hold circuit 32 to operate.
In addition to the ordinary video disk with FM video and audio signals recorded thereon in multiplexed form, there are other variations such as a digital audio video disk and a PAL type video disk. The digital audio video disk contains recorded FM video and audio signals overlaid with EFM digital audio signals in CD format. The PAL type video disk contains EFM digital audio signals as its only audio signal. If the digital audio video disk is subjected to on/off-track detection by the above-described VD-use on/off-track detection circuit, the circuit may malfunction due to the presence of the EFM signal recorded in multiplexed form. If the PAL type video disk containing the EFM signal as its only audio signal is handled in the same manner, the EFM signal causes the on/off-track detection circuit to malfunction. In both cases, normal detection of on- and off-track states is unavailable.